Use proper per-file GPL headers

[sandrobonazzola]

While reviewing the Fedora package, the automatic tool which check for license compliance detected:
Checking patched sources after %prep for licenses. Licenses
found: "GPL", "LGPL (v2 or later)", "GPL (v3 or later)", "Unknown or
generated". 97 files have unknown license.
Please add proper per-file GPL headers to ensure they're released under the right license.